In television broadcast such as satellite broadcast, ground wave broadcast, cable television (CATV), a wide frequency range is used. For example, an input frequency of a satellite broadcast tuner (indoor receiving device) ranges from 950 MHz to 2150 MHz, and an input frequency used in the cable television broadcast ranges from 52 MHz to 864 MHz. In the ground wave television broadcast, a frequency band similar to the cable television broadcast is used. This is based on the following reasons: a wide frequency is required in sending a television signal which has large information, and many programs are transmitted by using many channels so that a viewer can make a selection from many channels.
In a receiving device for receiving broadcast by using the wide frequency range, a circuit block (tuner, front end) which processes an antenna signal input converts a frequency used in the broadcast to a specific frequency (intermediate frequency)(in CATV, there is a case where conversion is performed twice). The received signal that has been converted into the intermediate frequency is subjected to a signal process such as amplification, filter, and modulation, so that images and voice signals are obtained. Upon converting the received signal, a local oscillation signal is required in a receiver.
In a front end of the receiver, a circuit block called a mixer performs frequency conversion. In the mixer, product of an input signal Asin ωRF by a local oscillating signal Bsin ωLO is outputted. An output Y of the mixer is as follows.
                    Y        =                ⁢                  k          ⁢                                          ⁢          A          ⁢                                          ⁢          B          ⁢                                          ⁢          sin          ⁢                                          ⁢                      ω            RF                    ⁢                                          ⁢          sin          ⁢                                          ⁢                      ω            RO                                                  =                ⁢                  k          ⁢                                          ⁢          A          ⁢                                          ⁢          B          ⁢                      {                                          0.5                ⁢                                                                  ⁢                sin                ⁢                                                                  ⁢                                  (                                                            ω                      RF                                        +                                          ω                      LO                                                        )                                            +                              0.5                ⁢                                                                  ⁢                sin                ⁢                                                                  ⁢                                  (                                                            ω                      RF                                        -                                          ω                      LO                                                        )                                                      }                              Here, k is a proportionally constant.
This expression shows that: it is possible to obtain a frequency component of a difference between the input signal frequency and the local oscillation frequency in accordance with the output of the mixer, so that it is possible to perform the frequency conversion.
In a case of a digital satellite broadcast, a zero frequency is used as the intermediate frequency (direct conversion mode). The local oscillation frequency in this case is required to range from 950 MHz to 2150 MHz as in a receiving frequency. Further, frequency conversion is performed at two oscillation frequencies having 90 degree phase difference so that original information is retained even though the local oscillation frequency is converted to a zero frequency. Then, the two outputs signals are used (they are called “base band signals).
In the cable television broadcast, a preferable distortion characteristic is required because broadcast is performed by using extremely large number of channels. Therefore, it is general to use a mode in which the frequency conversion is performed twice (direct conversion mode). At the first frequency conversion, a receiving frequency is converted to an intermediate frequency higher than the receiving frequency. Supposing that the intermediate frequency is 1000 MHz, a local oscillation frequency required in this case ranges from 1052 MHz to 1864 MHz.
A local oscillation circuit for generating the local oscillation frequency is required not only to provide a wider oscillation frequency range, but also to stabilize the frequency with respect to variation of a temperature or a power source voltage. Further, in a case of receiving the digital broadcast, it is necessary that a phase noise is preferable.
In order to satisfy these conditions, it is general to use an LC oscillation circuit containing a resonant circuit constituted of an inductor (L) and a capacitor (C). It is general to use a diode (vari cap diode, Variable Capacity, VC), whose capacitance is varied by a voltage between terminals, so as to vary the frequency. Thus, it is possible to obtain the oscillation circuit whose frequency is varied by a voltage. This is called “VCO” (Voltage Controlled Oscillator) in general.
Supposing that the inductance of the resonant circuit is L (H) and the capacitance of the resonant circuit is C (F), the oscillation frequency (resonant frequency) shown as oscillation frequency fo (Hz) of the LC oscillation circuit is determined in accordance with the following expression.fo=1/{2π√(LC)}Further, supposing that (a) an oscillation frequency in a case where capacitance C is Cmax (maximum value of C) is “fomin” and (b) an oscillation frequency in a case where capacitance C is Cmin (minimum value of C) is “fomax”, it is possible to obtain a relationship expressed as follows.fomax/fomin=√(Cmax/Cmin)That is, as a variable range of the frequency, a ratio between the maximum frequency and the minimum frequency (capacitance variation ratio) is identical with a square root of a ratio between the maximum value and the minimum value of the resonant circuit. As the ratio between the maximum frequency and the minimum frequency is larger, it becomes more difficult to realize such circuit. From the view point of the aforementioned example, the following setting is required: in a digital satellite broadcast tuner, 2150/950=2.26, and in a cable television tuner, 1864/1052=1.77.
In a conventional technique, a vari cap diode whose capacitance variation ratio is large is used so as to obtain these frequency variable ranges. Further, a voltage up to 30V is applied to the vari cap diode so as to obtain the capacitance variation ratio. In a case where a required variable range cannot be obtained, the inductor of the resonant circuit is switched, or a plurality of VCOs are provided so as to be switched.
As to precision in processing an integrated circuit, miniaturization has been promoted year by year, and a frequency characteristic of a transistor continues to advance. The frequency characteristic of the transistor is shown as a cut off frequency fT. The ft is required to be not less than 10 times as large as a frequency actually used. The ft of a transistor manufactured in accordance with recent bipolar, BiCMOS, RFCMOS processes is over 20 GHz. This characteristic is sufficient to manufacture a receiver front end of a television broadcast such as the satellite broadcast in accordance with these IC processes. Thus, development is promoted with respect to a receiver front end for a television broadcast, such as the satellite broadcast, bringing about a large market, so as to realize the receiver front end by using an IC. Particularly in a case of using an IC in which high frequency portions are entirely integrated, a relatively large front end module which is covered by a conventional metal case is replaced with the IC, so that this contributes to miniaturization and cost reduction of a broadcast receiver.
In realizing such high frequency IC, it is important to realize a VCO having a wide frequency range at a low cost as much as possible. This is because there occur the following problems.    (a) The capacitance variation ratio of a vari cap element which can be realized by using the IC is small.    (b) Q of an inductor which can be realized by using the IC is low.    (c) An area of the inductor which can be realized by using the IC is relatively large, so that this influences a price.    (d) There occurs a dispersion in the frequency due to dispersion in the IC process.